Network units such as switches and routers which include a multiplicity of ports for connection to other units or users are commonly made with a fixed number of ports in order to achieve efficiency of manufacture. It is known to ‘stack’ such units, by which is meant their connection in a cascade which enables them not only to receive and forward packets from their own ports but also to forward packets to other units in the stack if a destination port is on a unit other than the unit which first received the packets. Packets which are passed between the units for this purpose travel on the cascade connection, which may consist of ordinary links between cascade ports on the units but may be in more sophisticated form, such as described in GB patent 2365718. A unit needs at least one bi-directional cascade port which can communicate in the up and down directions (or at least two unidirectional ports for the same purpose) but may have more than the minimum number of cascade ports.
Units which are in a cascaded stack communicate between themselves so that the individual units are aware of which of the units are in the stack and for a variety of other purposes not directly relevant to the invention. Control of the units and communication between them may be ‘in band’, by means of control or management packets sent by way of the packet data path which constitutes the cascade but may be ‘out of band’ where the control information is sent by way of control messages on a separate control data path.
For a variety of purposes, it is desirable in a stack of units for a particular unit to be, in respect of at least some functions of the stack, a ‘lead’ or ‘master’ unit and for the other units to be ‘subordinate’ or ‘slaves’. The terms ‘master’ and ‘slave’ are not intended to connote a relationship wherein the ‘master’ unit performs all the relevant functions and the ‘slaves’ are merely completely driven by the master. For example, and is more particularly described here and after, an embodiment of the invention comprises a stack of units which are each capable of routing and bridging and are organized so that the unit which receives a packet on one of its external ports performs any necessary routing operation; however, where the packet has a destination port on a different unit, the packet is then bridged to the destination port. In this context, the ‘master’ will perform certain ancillary functions, such as an advertising function in a routing protocol, whereas the subordinate slaves need not.
It will be understood that ‘routing’ and ‘bridging’ are used in the ordinary senses wherein bridging relates to a switching operation occurring on the same LAN or sub-LAN and does not normally involve any change of the address data in the packet. On the other hand, ‘routing’ refers to a process wherein there is a change of network or sub-network and there is commonly a change of the address data in a packet, such as for example changing the destination MAC address.
The invention is particularly concerned with operation in, according to the OSI model, ‘layer 2’, which relates to media access control addresses, and ‘layer 3’, which relates to ‘network’ or ‘IP’ addresses.
The present invention is particularly concerned with the provision of ‘resilience’ against failure of a cascade connection between the units in the stack. In the event of such a failure, if units in both ‘halves’ or segments of the stack continue to provide full functional capability, such as for example full routing functionality, then two such units may exist on potentially the same network with the same identity, causing an ambiguous situation for any device wishing to communicate through them. An important example is where the stack performs an election to select a ‘lead’ or ‘master’ router and there is in consequence a supply of IP addresses from the lead or master router to routing interfaces in the other units of the stack as well as, preferably, the supply of MAC addresses to those router interfaces, as described in U.S. Pat. No. 7,123,615, issued to Ralph Wevman and Maurice Goodfellow on Oct. 17, 2006, commonly assigned herewith and incorporated herein by reference. As is described therein, the failure of an individual unit can be accommodated by a fresh election amongst the other units in the stack so as to provide, if appropriate, a single new lead or master unit. If however the cascade connection is lost and if both ‘halves’ of the stack continue to offer full routing functionality, then two logical routers can exist on potentially the same local network.